AMSTETTEN, Austria (CNN) -- Austrian police believe a 73-year-old man held his daughter captive in his cellar for the past two decades and fathered at least six children with her, according to police and state-run news reports Sunday.

The woman, identified as 42-year-old Elisabeth F., has been missing since 1984, when she was 18 years old, police said at a news conference.

The situation came to light earlier this month after her daughter -- a 19-year-old woman, identified as Kristen F. -- was hospitalized in Amstetten after falling unconscious, according to police.

She was admitted to a hospital in Amstetten, outside Vienna, by her grandfather with a note from her biological mother requesting help. Amstetten is a rural town about 150 km (93 miles) west of Vienna.

But police said a DNA test later revealed her grandfather, Josef F., was also her father, according to ORF, Austria's state-run news agency.

That sparked a police investigation, which revealed that Josef F. may have fathered at least six children with his daughter, forcing her and three of the surviving children to live in the cellar of his house, according to ORF's Peter Schmitzberger.

The children are now between 5 and 19 years old.

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Police are awaiting DNA tests to verify their relationship to Josef F., who faces arrest for "severe crimes against family members," according to police. So far, he has not given a statement to police.

Police spokesman Franz Polzer told ORF that the 73-year-old has led police to several hidden rooms in his cellar accessible only by an electronic passcode that he provided to police. Watch a report on the discovery »

On Sunday, police searched the hidden rooms where Josef F. admitted he kept his daughter and their children, Polzer told ORF. The rooms included sleeping quarters, a kitchen and a bathroom, which Josef F. told police he built, Polzer said. Neighbors told ORF they were shocked to hear the news, and had no indication such horrors were taking place in their town.

"One can't imagine how it could happen, how nobody could realize anything of what was going on in the cellar of this house," Schmitzberger told CNN. "It's quite unimaginable."

Acting on "a confidential tip," Amstetten police apprehended Josef F. and Elisabeth F. on Saturday near the hospital for questioning, according to a police statement. Once police assured the daughter that she would never have contact with her father again, "she was able to tell the whole story," Schmitzberger said.

Josef F. lived upstairs with his wife, Rosemarie F., who police said had no idea about her husband's other family living in the cellar.

Josef F. and Rosemarie F. had adopted three of the children that he had with his daughter, according to police. He told his wife that his missing daughter had dropped the unwanted children off at the house because she could not take care of them, police said.

The other three children -- Kerstin, 19; Stefan, 18; and Felix, 5 -- remained locked in the basement with their mother, according to police. None had seen the light of day during their entire time in captivity, police said.

After she was detained Saturday, Elisabeth F. gave police a "psychologically and physically disturbed impression," police said in a statement. She told them her story after she was assured her children would be protected from further harm.

She said her father began sexually abusing her at age 11. On August 8, 1984 -- weeks before she was reported missing -- her father enticed her into the basement, where he drugged her, put her in handcuffs and locked her in a room, she told police. For the next 24 years, she was constantly raped by her father, resulting in the six surviving children, she said, according to the police statement.

She told police she gave birth to twins in 1996, but one of the babies died a few days later as a result of neglect, and Josef F. removed the infant's body and burned it.

She told police that only her father supplied her and her children with food and clothing, and that she did not think his wife knew anything about their situation.

When Kerstin fell ill earlier this month, Josef F. apparently told his wife and the hospital that his "missing" daughter had dropped off the sick girl on his doorstep.

In an effort to find out what might be ailing 19-year-old Kerstin, the hospital asked the media to put out a bulletin requesting any information about the girl or her missing mother, attorney general Gerhard Sedlacek told NTV.

Sometime later, Josef F. brought Elisabeth F. out of the cellar, telling his wife that she had returned home with her two children after a 24-year absence, police said.

He took Elisabeth F. to the hospital to talk with doctors about Kerstin's condition, and at that point, authorities became aware of her situation, Sedlacek said.E-mail to a friend